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Mystery for the LCB collective.
adithorp - 1/8/22 at 01:56 PM

I'm flummoxed. Customer dropped his car in for service and MoT today. 67 plate Vaux' Mokka (petrol). No problems appart from a squeeking noise from the engine. Did the service (didn't hear the noise) and took it for MoT Passed no problem but the noise was now there once it was hot... ee ee ee eeee ee eee ee...

It sounds to all the world like an aux belt, tentioner/idler pulley, although or waterproof related noise and its coming from that area... so we check and can't trace it. A dab of silicone spay on the belt makes no difference. Stethoscope (ok a long screw driver) doesn't identify it from anything down there.

Then we switch of and the noise continues briefly after the engine stops. We double check and it's definitely after everything has stopped rotating. Sometimes it stops with the engine then does it a second or two later (for a second or two).

We did discover it dissapears (or greatly reduced) when the AC is on and the compressor running. It returned when the AC is off.

Any ideas?

big_wasa - 1/8/22 at 02:08 PM

Alternator ? As most now have a bearing so they free wheel once the engine stops.

coyoteboy - 1/8/22 at 02:13 PM

Mobile joints on the exhaust - either flexi sections or ball-socket type spring loaded ones. Often once they heat up they start to squeak in a high pitch and changing load on the engine can move it out of the frequency range that makes it squeak. I have a pug which waits until it's warm to start screeching and it drives me bananas. Even with a new gasket.

gremlin1234 - 1/8/22 at 02:24 PM

I think the next test should be:
load the alternator. - ie headlights, fog lights, heated rear window, heated seats / windscreen etc, to see if that changes the symptoms.

number-1 - 1/8/22 at 02:54 PM

Does the noise rise and fall with the revs range or does it stay the same?

I had a noise like that on the supercharged SSC stylus. Turned out to be a air leak in a throttle body gasket but however i was convinced it was the alternator bearing or belt before finding the real reason

loggyboy - 1/8/22 at 03:09 PM

Do they have a recirculating pump for coolant?
I know years back renault did it where by the coolant was kept moving for a a few secs/mins after turn off. Could be remote from belt area.

[Edited on 1-8-22 by loggyboy]

Slimy38 - 1/8/22 at 03:19 PM

quote:
Originally posted by number-1
Does the noise rise and fall with the revs range or does it stay the same?

I had a noise like that on the supercharged SSC stylus. Turned out to be a air leak in a throttle body gasket but however i was convinced it was the alternator bearing or belt before finding the real reason


Air leak would be my guess too, something pressurised with the engine running, the heat opening up the hole/leak then when the engine switches off the pressure can dissipate. Although I'm not sure how to explain the AC compressor affecting it (unless it's climate control related and the AC being on means that airflow is being diverted elsewhere?)

adithorp - 1/8/22 at 03:34 PM

quote:
Originally posted by big_wasa
Alternator ? As most now have a bearing so they free wheel once the engine stops.


I thought of that but I can see the armature has stopped when the post switch off squeek happens.

adithorp - 1/8/22 at 03:36 PM

quote:
Originally posted by coyoteboy
Mobile joints on the exhaust - either flexi sections or ball-socket type spring loaded ones. Often once they heat up they start to squeak in a high pitch and changing load on the engine can move it out of the frequency range that makes it squeak. I have a pug which waits until it's warm to start screeching and it drives me bananas. Even with a new gasket.


Doesn't seem to be. Manipulating the exhaust has never effect.

adithorp - 1/8/22 at 03:39 PM

quote:
Originally posted by number-1
Does the noise rise and fall with the revs range or does it stay the same?

I had a noise like that on the supercharged SSC stylus. Turned out to be a air leak in a throttle body gasket but however i was convinced it was the alternator bearing or belt before finding the real reason


It's a repetitive noise not a constant one. Sounds rotational and not like an air/vac leak.

adithorp - 1/8/22 at 03:40 PM

quote:
Originally posted by gremlin1234
I think the next test should be:
load the alternator. - ie headlights, fog lights, heated rear window, heated seats / windscreen etc, to see if that changes the symptoms.


Done all that and no difference.

adithorp - 1/8/22 at 03:41 PM

quote:
Originally posted by loggyboy
Do they have a recirculating pump for coolant?
I know years back renault did it where by the coolant was kept moving for a a few secs/mins after turn off. Could be remote from belt area.

[Edited on 1-8-22 by loggyboy]


Thought about that but it doesn't have one. Electric 'stat maybe??? But what effect would the ac have?

[Edited on 1/8/22 by adithorp]

gary gsx - 1/8/22 at 04:52 PM

My son had a Corsa that had a squeaking sound that didn’t seam to go with the engine revs high or low just squeak squeak turned out to be a loose spark plug being muffled by the coil pack


Gary

obfripper - 1/8/22 at 05:20 PM

I've had similar noises on a 1.4tsi audi after changing the timing belt. Took off the aux belt but noise was still there randomly sounding like it was from the a/c pump/alternator but couldn't rule out the belt i'd just fitted. Out of frustration pulled off one of the pcv hoses and the noise stopped! Replaced 2 of 3 pcv valves on the engine and all was good again.

I have had the pcv in the rocker cover fail on the 1.2/1.4/1.6 vauxhall engines, never noticed any significant noise but they would suck in air and cause idle & mixture problems. Covering the little hole in the pcv cover would change the idle, or a spray of brake cleaner in that area would make the engine hunt.

Dave

adithorp - 1/8/22 at 05:52 PM

I wondered about pcv Dave but why would switching AC on stop it?

Customer has taken the car now. He's not had it long and still has warranty on it from the dealers so I guess they're going to play parts darts with it before I see it again.

theconrodkid - 1/8/22 at 06:53 PM

does sound like a vacume leak to moi

obfripper - 1/8/22 at 07:02 PM

The audi did exactly the same, would go quiet with a/c on seemingly caused by the rise in rpm. The noise did carry on just past the engine turning off, but not to the point that it was obviously a vacuum build up, i also noticed removing the oil cap did also silence the noise while trying to isolate exactly what was occuring.

I had to dig out the technical info for the engine to work out the operation of the pcv system as multiple valves stop the crankcase pressurising under boost while allowing the normal operation during idle, i'm sure similar ideas are used on the vauxhall when it's turbo'd.



Dave

nick205 - 1/8/22 at 08:30 PM

I'd be going with a pressure leak somewhere. Maybe a loose/split hose somewhere or even an a knackered hose clamp.

ReMan - 2/8/22 at 10:25 PM

Heres my guesses
Could it be the main cabin air blower fan?
aux belts off to eliminate many things?
Main rad fan slip drive thing!?
Front crank seal?

adithorp - 2/8/22 at 10:58 PM

quote:
Originally posted by ReMan
Heres my guesses
Could it be the main cabin air blower fan?
aux belts off to eliminate many things?
Main rad fan slip drive thing!?
Front crank seal?


1. No, noise is definitely o/strontium engine bay. Heater fan is behind glovebox

2. Didn't bother as noise continues after everything has stopped turning (or comes back briefly a second or two after it's stopped)

3. Slip drive thing??? But no . Its electric fan and noise happens when it's not running.

4. See 2

I think Daves probably right and it's a pcv valve acting like a whistle.